Current VIP technology used in refrigeration units and the like generally comprises a single panel of insulation material formed typically of fumed silica, mineral fibre or other microporous material core, which may or may not contain infra-red opacifiers, the core wrapped in a flexible, gas-tight envelope to which a vacuum is applied before sealing.
Thermal conductivity properties of VIPs are typically of the order of about 0.005 W/mK. All thermal conductivity values referenced herein are those determined under BS EN: 12667:2001 unless expressly indicated otherwise. mW/mK are the units for thermal conductivity and are milliwatts per meter Kelvin.
EP2607073 describes a VIP comprising a composite core material formed of glass fiber wool and glass fiber board and an outer skin material having a layered structure comprising a surface protective layer, a metal barrier layer, and a bonding layer from the outside to vacuum-package the core material. The object of EP2607073 is to provide a VIP which exhibits a long-term durability of 10 years or more. However, despite starting from an initial thermal conductivity of approximately 2.4 mW/mK, the predicted thermal conductivity after 2 years was approximately 6.4 mW/mK. By incorporating a glass fiber board, although the initial thermal conductivity is satisfactory, the lifetime of the board decreases rapidly over time.
JP2002310384 describes a VIP comprising a glass fibre insulating core and reinforcing members which are laminated to the insulating core. The reinforcing members are for example made of stainless steel or polyethylene terephthalate. The presence of the insulating core improves the physical appearance of the VIP but its incorporation leads to a decrease in thermal performance, as its inclusion results in an increase in thermal conductivity of the VIP.
JP2006029686 describes a VIP comprising an insulating core accommodated in a gas barrier container. The container comprises a multilayer resin film comprising an aluminum vapour deposition film and a reinforcement layer comprising ethylene vinyl alcohol copolymer and inorganic-type material.
JPH08178176 describes a VIP comprising an insulating core contained in a breathable bag which is housed between reinforcing plates which prevent bending or twisting of the VIP.
Notwithstanding the state of the art, there remains a long felt, yet unmet need to provide VIPs with improved thermal conductivity and a long-term durable lifetime.
It is known that the thermal conductivity of a VIP product may be improved by reducing the density of the core, which has the added benefit of reducing the production cost of the VIP. However, a reduced density core is less robust and more prone to breakage and thus is more difficult to handle during the various production stages.
Thus, while it is desirable to reduce the density of the core as much as possible in order to improve thermal performance, the more the density is reduced, the more compromised are the handling properties of the core and any VIP made from it.
For example, conventional VIPs made from powder insulation cores with core density values of lower than about 165 kg/m3 have poor dimensional stability, the edges of the panels collapse and the panel envelope wrinkles. The result is an inferior product, which tends to warp over time and has an aesthetically displeasing appearance. A balance must therefore be struck between thermal performance and dimensional integrity on the one hand and ease of handling, on the other hand.